Quad-rotor aircraft are an increasingly common form of aircraft used in a variety of small to mid-size designs of primarily unmanned aircraft. Quad-rotor aircraft are desirable for their ability to take-off and land vertically and the simplicity of their design. Because they are controlled entirely by varying the application of power to the four rotors, quad-rotor craft can use fixed rotors and do not need variable-pitch rotors such as those found in helicopters, greatly reducing both the complexity and cost of the aircraft.
However, quad-rotor aircraft are unable to cover ground as efficiently, or quickly, as fixed-wing aircraft when travelling over long distances, and are unable to loiter over an area as efficiently as a circling fixed-wing aircraft can at a reduced power-setting. These deficiencies limit the general usefulness of the quad-rotor design.
Likewise, fixed-wing aircraft are unable to land or take-off in as short a distance as quad-rotor craft, which in turn limits the general usefulness of the fixed-wing design. Thus, a clear trade-off in endurance and versatility exists between the two designs, and an operator must often choose to accept the inherent limitations of either design over the other when choosing the kind of aircraft best suited to the demands of a given operation. Other drawbacks and limitations of existing devices are also present.